Problem 2: A 70 kg man runs up a long flight of stairs in 4.0 seconds. The vertical height of the stair is 4.5 m. Calculate his power.

Solution

Theory: When the man goes up stairs, he is doing work by lifting his own weight to a vertical height. This work is equal to the gain in the potential energy of his body. Power is the rate of doing work (P = W/t). Hence, the gain in potential energy will be calculated which is equal to the work done and then divided by the time he takes will give us the power of the man.

Given                   Mass of the man = 70 kg     OR   weight of the man = 70 × 9.8 = 686 N

                              Vertical height = 4.5 m,                  Time taken = 4.0 s

Required             Power of the man

Now gain in the P.E = mgh = 686 × 4.5 = 3087 J = the work done the man do go upstairs.

Power = work done/time = P = 3087/4.0 = 771.75 = 772 = 7.7 × 10² watts